The shape and structure of branches attached internally to the stem (knots) for loblolly pine ( Pinus taeda L.) trees were modeled. Data on knot shape were obtained from the dissection of branches taken from 34 22-yr-old sample trees growing under ten different initial spacings. A total of 341 branches located below the live crown were dissected in the radial/tangential plane. Afterward, a procedure was implemented to reconstruct the branch diameter perpendicular to the branch pith. This information was used to develop a model for representing knot shape, which assumed that the live portion of a knot can be modeled with a one-parameter equation and the dead portion by assuming a cylindrical shape. To study the variability in shape of individual knots (live portion), the model was fitted to 218 branch profiles using nonlinear mixed-effects modeling techniques. A graphical analysis indicated that the random-effects parameter was related to branch diameter. Thus, branch diameter was included as a predictor variable to reduce between-individual variability in knot shape. Reconstructed knots with smaller diameters were more cylindrical; those with larger diameters were more parabolic or conical in shape. Analytical expressions were derived for estimating the volume of knots (live/dead portions) for three types of branch conditions on simulated trees: 1) live branches; 2) nonoccluded dead branches; and 3) occluded dead branches. The knot model assumes a substantial simplification of branch morphology, but should be useful for representing knots as 3-D entities in the stems of loblolly pine trees.